1. Field of the Invention
The present invention relates generally to traffic monitoring, and more particularly to recording information about moving vehicles. Specifically, the present invention relates to frequency-domain techniques for processing a Doppler signal from a Doppler-shift radar transceiver in order to determine with a high level of confidence whether a vehicle is exceeding a specified speed limit.
2. Description of the Background Art
Automatic traffic monitoring systems have been operated in the United States since about 1986. Automatic traffic monitoring systems are used for detecting speed violations, red-light violations, and drunk-driving (DUI) violations. The systems reduce traffic accidents, and generate "fine" revenue that pays for the cost of installing and maintaining the systems.
For detecting and recording speed violations, an automatic traffic monitoring system includes a Doppler-shift radar transceiver and a high-speed camera triggered by the radar transceiver. The high-speed camera takes a picture of the front of the approaching vehicle, and a second camera may be used to take a picture of the rear of the vehicle after the vehicle passes the location of the traffic monitoring system. An alphanumeric data block containing the measured vehicle speed, date, time, and location is also recorded on a portion or edge of the photograph. A video camera may be used to record a motion picture of the traffic, and the alphanumeric data has been interleaved with the video picture on a video display and recorded on a video cassette recorder (VCR). A display may also be deployed at the monitoring site to display the measured speed to those drivers which were photographed. Detailed information from each monitoring session is also recorded on computer diskette. The detailed information can be used in traffic studies, and also in traffic court to demonstrate that a motorist was indeed traveling faster than surrounding traffic.
The accuracy of speed measurement is a primary consideration in the design of any traffic monitoring system intended to detect speed violations. It is well-known that radar reflections from receding traffic can interfere with reflections from approaching traffic. To discriminate between approaching and receding vehicles, the radar transceiver employs a quadrature-detector providing an indication of the direction of travel of the vehicle with respect to the radar transceiver. Reflections from receding vehicles are ignored. Moreover, as a vehicle passes through the radar beam, more than a hundred speed measurements are taken. Any bad readings are recognized and discarded. If the distribution of measurements is not found to match a cosine decay, the measurements are deemed to be inconclusive.
Another important consideration in the design of a traffic monitoring system for detecting moving violations is to obtain a clear picture of the offending vehicle. A high-powered flash fitted with a ruby colored filter has been used to illuminate the vehicle and driver. During the day, the flash helps to reduce glare so that the driver is more visible in the photograph. At night, the flash provides the light required to illuminate the entire scene for the photograph. The flash is also triggered when the vehicle is at a rather precise distance from the camera. The radar beam, for example, is directed by an antenna having a conical horn and a 7 inch dielectric lens providing a narrow half-power beam-width of 5 degrees, and the beam intersects the roadway at a 221/2 degree angle. The effective range of the radar is 300 feet, so that the vehicle's location is precisely known when it first enters the radar beam. The first few speed measurements determine if a photograph should be taken. A computer analyzes the complete group of measurements and compares them to an expected cosine decay. The speed of the vehicle is imprinted on the film only when the complete set of measurements has been verified.
Due to the loss of right resulting from a conviction for a speeding violation, there is always a desire for improvement in the level of confidence associated with the determination of a vehicle's speed. One troublesome source of inaccuracy is the possibility of multiple vehicles being present in the radar beam, for example, when one vehicle passes another. In such a situation, the Doppler signal may have an average frequency that is representative of neither of the speeds of the vehicles. Systems that measure the frequency of the Doppler signal by counting level crossings of the Doppler signal measure the average frequency of the Doppler signal and therefore may give an erroneous speed measurement. A more troublesome source of interference is reflections from tires. A reflection from the top of a tire, for example, indicates a speed twice as fast as the speed of the vehicle.
Electronic tracking filters have been used to select Doppler signals in different frequency ranges. As described in John Hewer, "High Technology Instrument Foils Hasty," Canadian Electronics Engineering, August 1979, pp. 30-31, such a tracking filter can be used in traffic monitoring system mounted in a moving surveillance vehicle in order to discriminate between a Doppler signal frequency representing the speed of the moving surveillance vehicle, and a Doppler signal frequency representing the speed of another vehicle relative to the speed of the moving surveillance vehicle. Mr. Hewer says that the tracking filter also overcomes the effect of weak Doppler signals and permits the system to pick the lowest or highest frequency in any given spectral range without the need for large signal amplitude priority.